The use of the fabricated open web steel joists is very common in the building and construction industry. Such joists may be provided over a wide range of flexibility as to design and weight of components, depending on the intended use. However, it is incumbent upon the manufacturer to provide the fabricated joists to its customer as economically as possible, and preferably with a minimum of set-up time and costs in the factory. Even after the set-up has been made, it is a feature of the present invention that there is minimum handling, and maximum ease of fabrication, thereby minimizing worker fatigue.
To accomplish the fabrication of a more superior joist, the provision is made by the present invention for the joist to be assembled in a vertical orientation--that is, with top and bottom chord of the joist horizontally disposed one above the other. Moreover, the invention provides for the inversion of the joist during its assembly process, while still in a jig, for even greater ease of manufacture and for better quality welding.
The fabricated open web steel joist of the present invention has a top chord, a bottom chord, and a web extending between them. Each of the top and bottom chords comprises a pair of generally L-shaped angles which are arranged in back-to-back relation. One leg of each "L" is horizontally disposed, at the upper extremity or the lower extremity of the joist, and the other leg of each of the "L"'s are arranged, in pairs, in opposed vertically disposed manner.
The web comprises a series of pre-formed bar sections, each of which has the general configuration of a "W". The ends of the sections are located in the top chord, in adjacent positions except at the outer ends of the joist. Each "W" spans two panels along the length of the joist; so that, where necessary because of there being an odd number of panels required in the length of the joist, a "V" may be inserted between two of the W-shaped sections within the web--usually at the centre thereof. The joist is welded, where each end and each apex of each "W" and "V" which is included in the joist is welded to each of the vertically disposed faces of the L-shaped angles that form the top and bottom chords of the joist.
The subject construction is clearly distinguished over the prior art, which in respect of open web joists generally require the use of serpentine web bar members. Such prior art includes U.S. Pat. No. 1,915,424 issued in June, 1933 to KERR; U.S. Pat. No. 1,983,632 issued in December of 1934 to MILLER et al; U.S. Pat. No. 2,624,430 issued in January, 1953 to McCORMBER et al; and U.S. Pat. No. 3,639,962 issued in February, 1972 to GOODER. Each of the prior art patents noted above shows that a continuous web member is used, formed in a serpentine manner, and that the web member extends substantially along the full length of the joist. This creates a number of problems, particularly during assembly of such joists. Indeed, one such problem is that the end compression leg of certain prior art joists is vertical or nearly so, making its welding into place difficult to achieve--thereby usually dictating that the joist is turned on its side for fabrication.
For example, handling the material of the prior art webs is difficult, due to their own length, and due to the slenderness of the web member which may be formed of relatively light material. Not only may a prior art web be accidentally distorted, the mere act of welding it--usually when horizontally disposed because otherwise it is too difficult to handle--will cause deformation.
The process of fabrication of open web steel joists, according to this invention, includes the steps of:
(a) pre-forming a plurality of bar sections, each in the general form of a "W", each of which will cover two panels along the length of the joist; and in the event that an odd number of panels is required, pre-forming a bar section in the general form of a "V";
(b) assembling the top chord components, bottom chord components, and pre-formed bar sections in a jig, so that the joist is completely assembled as to the required components for its fabrication, and clamping the jig to maintain the assembled components in place; and
(c) welding the clamped components so as to form a fabricated joist.
Generally, step (c) is carried out in three individual steps, namely:
(c)(i) welding the ends and apexes of the pre-formed sections which are between the opposed faces of the top chord;
(c)(ii) inverting the still-clamped assembly of the components and jig; and
(c)(iii) welding the now upper located apexes of the pre-formed sections which are between the opposed faces of the bottom chord.
Because of the inversion of the joist during its assembly, the present invention provides for a much more economical assembly or fabrication line for welded open web steel joists, reducing handling and labour costs, and increasing flow-through time by the simple expedient of never reversing the direction of flow of the joist while being fabricated, or even rotating the joist in an end-to-end fashion. Still further, the present invention provides, by assembly and welding of the joist in a vertical position, for a joist having very little "sweep" or side-to-side distortion along its length. Even further, by assembly using a jig which supports the joist, and which inverts it, the joist can be manufactured with essentially no camber from end to end, or with a pre-designed camber as required. The support of the joist in the jig may be at the centre point, but more usually is at the outside quarter points (i.e., one-quarter of the length of the joist in from each end). This manner of support assures that there will be no significant sag in the joist, because the turning moments in the joist about the support points are balanced.
The welding technique used, according to the present invention, is designated as "vertical-up", and as such it is a technique which is easily performed by welders in a standing position without having to distort their own bodies or to weld above the welding gun. More particularly, the use of a vertical-up welding technique permits both sides of the web element to be welded at the same time, thereby eliminating or effectively precluding undue distortion in the joist. The weld wire can be inserted into the space between the opposed vertically disposed faces of the chord member which is in the upper orientation at any time, and a vertical-up weld can be made on each side of the apex or adjoining ends of "W"'s within the web of the joist.
By using preformed web sections, all of which are symmetrical in the form of a "W" (or, one-half of the symmetry of a "W" when in the form of a "V"), the handling and placement of the web members is very efficient. Because the overall height and length of each web member is the same as those of all other web members (or one-half, in the case of a "V"), the jig may be arranged with equal panel lengths along the entire length of the joist, with each W-shaped web member covering two panel lengths. By virtue of the design of the web members, they may be so placed in an accurate manner that no significant accumulated error occurs along the length of the joist. Moreover, even though it is possible to design a joist having varying panel spans so as to take advantage of low shear requirements at differing points along the joist, the cost of calculating and re-setting the machinery to bend the rod material which forms the web is generally far greater than the savings that may be accomplished by using slightly less material.
When a joist is designed in keeping with the present invention, in a simple manner such that its fabrication reduces the number of manufacturing steps and operations to a minimum, and such that there is no backtracking or reversal of the direction of flow of the material during fabrication of the joists, then significant economies may be achieved. Moreover, economies of the savings of material can be achieved if a joist requirement varies from another joist requirement simply in the load bearing capacities, by substituting lighter chord material and/or bar material of a smaller cross section, but still having the same panel lengths and therefore the same pick-up points and jig points in the tooling. This reduces the cost of cutting and forming the bar material, as well as the cost of handling the pre-formed web sections.
For example, balanced web sections that are each generally in the form of a "W" can be suspended from their central apex over a single support, and many such pre-formed sections may be moved at any one time. Sections in the form of a "V", when needed, may also be easily handled by suspending them in an inverted condition from a single support. Of course, all of the included angles between the legs of a "W" or a "V", and therefore also the included angle between the endmost legs of adjacent bar sections, are equal in any one joist design. That being the case, the joist design is easily calculated by knowledge of its load bearing requirements, and by designing the joist as to the top and bottom chord members and as to the height of the web and the cross sectional area of the web section components (which may comprise tension or compression members under load), as well as the height of the joist. All of those matters having been decided, the length of the joist may be divided into a convenient number of panel lengths, which may be an even number or an odd number. As noted, if the number of panel lengths is even, then only W-shaped sections would be used; if the number of panel sections is odd, then one V-shaped web section would be used and would be placed along the length of the joist, generally at the centre thereof.
Because the joist is fabricated by welding, using a gently sloping vertical-up welding technique, which may be done at a convenient height, the welders who are doing the actual fabrication will experience a minimum of fatigue. Also, the welding steps using vertical-up techniques mimimize the localized heat distortion of the joist components as noted above, so that the sweep (sideways distortion or bowing) of the joist is minimized. The current standards of sweep require that the sweep be no greater than 1/500th of the joist span; and where the joists are being placed in such a manner that steel erectors must rely on the joist placement for connecting or placing yet other members, particularly such as galvanized steel decking, on them, it may be dangerous or unacceptable practice if the joists have too much bow or wander in them. Using joists of the present invention, however, accurate centre-to-centre distances of adjacent joists may be maintained, thereby realizing yet further economies in erection of the structure being built, as well as assuring sound engineering structures which are in keeping with the design drawings for that structure.
The usual manner of fabrication of joists according to the present invention presumes that the web bars are cut and formed in a bender which may be dedicated for that purpose. Of course, as mentioned above, the amount of bend as to the included angle and as to the height of the web section may vary from joist design to joist design, with minimal tool set-up time required. Similarity, the L-shaped chord angles that comprise the top and bottom chords of the joist when assembled, would be precut and spliced where necessary to the appropriate length; and they may well have the bearing shoes which are required for the joist attached to them, prior to their delivery to the joist jig.
By another aspect of the present invention, the joist jig is normally supported as noted above on a positioner and inverter, thereby effectively eliminating sag in the joist. In any event, by supporting the joist jig and the components of the joist as noted, a pre-calculated camber for the joist may be accommodated. At the same time, using a joist positioner and inverter, together with the jig and clamping arrangements to maintain the components in place until the joist is fully fabricated, allows the positioner and inverter to be loaded from one side and unloaded from the other side without the requirement to rotate or upend the joist.
Obviously, fully welded joists, when removed from the joist inverter, may be carried singly or in bundles for further processing including pickling where necessary, paint tank dipping, draining, tagging and marking, and loading onto trailers for delivery to the job site.
The method of fabrication according to the present invention, therefore, allows for material handling using generally standard equipment which would normally be found in a steel fabricating shop, such as overhead cranes and jib cranes, as well as easily fabricated jigs, buggies, racks, and tables.
It has been determined that, by using vertical-up welding techniques, higher welding voltage and current can be employed without creating a risk of damage to the L-shaped angle sections that comprise the chords of the joist. Moreover, if the welding were to be carried out horizontally with the joist on its side rather than in the vertical-up manner, more handling steps would be required, at the costs of both labour and time.